The change from temporal to cause is one of the well-known grammaticalization paths. This article analyses the change in the Estonian conjunction kuna, &#8216;while; as, because&#8217; from temporal to causal one as well as the attitude of the Estonian language planners towards this change. This change has mainly taken place during the twentieth century. The end of the nineteenth century and the beginning of the twentieth century witnessed the prevailing use of the temporal use of kuna. However, during the latter part of the 20th century the use of kuna began to shift, and the use of causal kuna began to dominate. As an intermediary stage between the temporal and causal usages, kuna was used as an adversative temporal conjunction. The impact of this shift in usage of kuna as a causal conjunction has also slightly changed the use of other Estonian causal conjunctions. Initially, Estonian language planners adopted a strict attitude towards the change of kuna, but language planning could not halt this change. For example, editors continued to cross out the causal use of kuna, but they could not increase the temporal use, and this usage of the conjunction hardly appears in the written texts of the 1970s. However, the change was finally accepted in the 1990s.